This invention relates generally to a method for generating magnetostrictive vibration by using a novel composition of a magnetostrictive element comprising sintered cobalt-containing magnetite, and a magnetostrictive vibrator for use therein.
In general, ferrites for magnetostrictive vibrators have the advantageous feature of higher electrical resistance at frequencies in the ultrasonic region and, therefore, less eddy current loss, in comparison with metal magnetostrictive vibrators. For this reason, ferrites have been widely used as vibrators of ultrasonic equipment, the principal examples of which are ultrasonic washing machines and ultrasonic fabricating machines and tools.
The ferrites used at present for magnetostrictive vibrators are all materials having Ni ferrite (more exactly, which further contain Cu, Co and so on as minor components but hereinafter are referred to merely as Ni ferrite) as their principal constituent, but these materials are of complicated composition and are accompanied by the problem of high production cost due to high cost of the starting materials.
Furthermore, a Ni ferrite used for magnetostrictive vibrators is disadvantageous in that the absolute value of the saturated static magnetostriction thereof is not very great, whereby a vibrator produced therefrom does not necessarily possess ample performance as a powerful ultrasonic vibrator. Furthermore, the utilization thereof is restricted by its negative static magnetostriction.
On one hand, magnetites have been known as exhibiting magnetostrictive property, and attempts have been made to use sintered magnetites as magnetostrictive vibrators. However, these materials are not being used at present because of certain difficulties, the principal examples of which are as follows.
1. A product of sufficiently high density as a sintered material cannot be obtained, and for this reason, mechanical strength required for continuous magnetostrictive vibration cannot be acquired by the product.
2. The electrical resistance of a magnetite is of the order of 10.sup.-2 ohm-cm., which is substantially low for a ferrite, and the eddy current loss thereof cannot be therefore neglected.
As a result of our studies for overcoming the above described difficulties, we have previously developed a method for producing a magnetite of improved magnetostrictive properties which can be adapted for use in the production of magnetostrictive vibrators. The method comprises correctly regulating the starting iron-oxide material and strictly controlling the sintering conditions (as disclosed in U.S. patent application Ser. No. 484,869 entitled "Production of Ferrite for Magnetostrictive Vibrators" filed Mar. 6, 1976, by Yumi AKIMOTO et al.).